1. Field of the Invention
The present invention relates generally to controlling an oscillator or a phase-delay device, and, more particularly, to controlling such devices in a phase control circuit.
2. Related Art
Phase control circuits are used to generate signals which bear a given frequency or phase relationship to a reference signal. A signal of this kind may, for example, be generated by dividing the output signal of an oscillator in a given ratio of division, in which case it can be arranged in this way that the frequency of the output signal from the oscillator is a multiple of the frequency of the reference signal, which multiple depends on the ratio of division selected. In communications technology for example, this can be employed to allow a plurality of stable frequencies to be generated for different transmission channels with the help of only one, or a few, reference frequencies.
It is also possible for what is controlled in a phase control circuit to be not an oscillator but a phase-delay device, to which a signal is applied which is of the same frequency as the reference signal. The output signal from the phase-delay device then forms the controlling signal, in which case a signal which bears a given phase relationship to the reference signal can be generated in this way by means of the phase control circuit when for example fixed time lags are required in circuits.
A known phase control circuit of this kind is shown in FIG. 5. In this phase control circuit, a phase detector 3, which may also be a phase/frequency detector, compares the phases of a reference signal 21 and a controlling signal 22. The phase detector 3 generates a signal which controls current sources within a charge pump 4. The charge pump 4 generates two signals 5, 6 which together form a differential output signal which is applied to a loop filter 7. At the output of the loop filter 7 there is a differential output signal 8 which is connected to a control means 9 in which a control signal 18 to control an oscillator 1 is generated as a function of the output signal B. The output signal from the oscillator 1 is divided down by a divider 2, which may in particular be adjustable, to generate the controlling signal 22. With the phase control circuit settled to a steady state, the frequency of the reference signal 21 is the same as the frequency of the controlling signal 22, which means that the output frequency of the oscillator 1, divided by the ratio of division of the divider 2, is precisely the frequency of the reference signal 21. The phase control circuit shown in FIG. 5 is used to generate a specific frequency and is therefore provided with an oscillator 1 having a downstream divider 2. Alternatively, there may be provided in place of the oscillator 1 a phase-delay device to which a signal of the same frequency as the. reference signal 21 is applied and whose output signal directly represents the controlling signal 22. There is no divider in this case. In an arrangement of this kind having a phase-delay device, the phase control circuit is used to generate a signal which has a defined phase lag relative to the reference signal 21.
To obtain a desired output frequency from an oscillator 1 or a desired phase lag from a phase-delay device, a given control signal 18 is required. To enable the phase control circuit to be operated at a large number of different operating points, provision is made in the control means 9 for the signal 18 to be generated by adding an adjusting component 20 to a controlling component 19, with the controlling component 19 being generated as a function of the output signal 8 and the adjusting component 20 representing a manually adjustable offset or direct component for the control signal 18. Hence, it is known for there to be provided in the control means 9 a current or voltage source 13 by which the adjusting component 20 can be adjusted to suit the relevant requirements to be met by the phase control circuit. It is a disadvantage that this makes it necessary for the phase control circuit to be specifically adjusted to suit the particular purpose for which it is being used. Also, the adjustment by means of the current or voltage source 13 may under certain circumstances have to be changed if there is a change in the operating point of the phase control circuit.